1. Field of the Invention
The present invention is in the field of non-dispersive infrared (NDIR) gas analyzers of a type typically used to measure the concentrations of unwanted or combustible gases so that an alarm can be given when their concentration approaches a harmful or dangerous level. More specifically, the present invention relates to a comparatively small apparatus having no moving parts and capable of measuring the concentration of one or more specified components in a mixture of gases.
2. The Prior Art
The NDIR technique utilizing the characteristic absorption bands of gases in the infrared has been widely used in the gas analyzer industry for the detection of these gases. Such gas analyzers utilize the principle that various gases exhibit substantial absorption at specific wavelengths in the infrared radiation spectrum. The term "non-dispersive" as used herein refers to the apparatus used, typically a narrow-band optical or infrared transmission filter instead of a dispersive element such as a prism or diffraction grating, for isolating for purposes of measurement the radiation in a particular wavelength band that normally coincides with a strong absorption band in the absorption spectrum of a gas to be measured. The NDIR technique offers a number of distinct advantages over previous methods that use the principle of heat transfer based upon radiation absorption by certain gases. These advantages include speed of response, measurement stability, greater sensitivity and simpler implementation.
Over the years a large number of measurement techniques based upon the NDIR principle for the detection of have been proposed and successfully demonstrated. In one such gas analyzer shown and described in U.S. Pat. No. 3,793,525 by Burch, et al., a beam of infrared energy emanates from an infrared source and passes through a sample chamber containing an unknown gas mixture. Before reaching an infrared detector, the beam is passed through one or more narrow band-pass filters, which may be mounted on a filter wheel. Typically, each filter only passes radiation at the characteristic absorption wavelength of a particular gas of interest. Another filter may also be used as a reference filter at a wavelength close to, but not overlapping, the characteristic absorption wavelength of any of the gases present in the sample cell. This type of gas analyzer requires the generation of some type of synchronizing signal in order to coordinate the operation of the signal processing circuit with the rotation of the filter wheel.
Another type of NDIR gas analyzer is shown and described in U.S. Pat. No. 3,811,776 by Blau, Jr. It incorporates (in addition to the infrared source, sample chamber, narrow band-pass filter and detector) a reference cell (a gas cell containing the gas of interest, e.g., CO.sub.2) and an identical cell evacuated or filled with a gas that is transparent at the wavelength used (4.26 microns for CO.sub.2) such as N.sub.2. These two cells alternately are moved into and out of the radiation beam. Since a sample chamber is placed in series with these cells, the alternate introduction of the absorbing and nonabsorbing cells into the radiation beam creates, respectively, a reference detector signal and a sample detector signal whose ratio is used to determine the gas concentration in the sample chamber. Unlike the configuration described in U.S. Pat. No. 3,793,525 alluded to earlier, which utilizes two interposed optical filters to create a sample and reference detector signal, the Blau configuration takes advantage of the principle of nonlinear absorption by the gas to be measured (as discussed in U.S. Pat. No. 4,578,762 by Wong) in order to create the reference and sample signals.
Another improved type of such gas analyzer is shown and described in U.S. Pat. No. 4,694,173 by Wong. This gas analyzer has no moving parts for effecting either the interposition of optical filters or absorbing and nonabsorbing cells to create both a sample and a reference detector signal as in the NDIR gas analyzers described earlier.
All of the NDIR gas analyzers described above for the measurement of the concentrations of one or more gases in a mixture perform well functionally and have contributed overwhelmingly to the overall technical advancement in the field of gas analysis during the past two decades. They are widely accepted in both the medical and industrial communities. Despite their undisputed success over the years, there remain quite a number of applications, primarily in the industrial sector, where these NDIR techniques are still too complex, and hence too costly, to be taken advantage of. One such example is the methane gas detector for the miners. The ideal solution here is a small, very low cost and battery-operated methane gas sensor mountable directly below the headlight on the miner's helmet. In the event the miner encounters a methane gas pocket during excavation in the mine, this particular sensor can detect a dangerous level of the gas much sooner than the current setup in which a relatively bulky methane analyzer is normally located many feet behind the working miners. Furthermore, such a helmet-mounted methane gas sensor allows the alarm to be placed inside the helmet and close to the miner's ears thereby avoiding the tragic possibility that the alarm from a more remote methane analyzer might be drowned out by the machine noises in the mine.
Another example is the commonplace household fire sensor. Fire sensors in use today in almost all public buildings and private dwellings are in essence smoke detectors as they only detect the smoke resulting from a fire. These sensors are compact and low cost, but they have been known to generate frequent and annoying false alarms. It is generally believed that the detection of an elevated level of carbon dioxide gas as a result of the combustion process taking place in any fire is a better alternative to the smoke detector in terms of false alarms. However, implementation of such a fire sensor using NDIR techniques presently available is far too complex and costly to serve as a viable alternative.
In view of these situations and the incessant drive for better and lower cost gas analyzers, new techniques are constantly being proposed and introduced all aiming at coming up with still better solutions. Maiden in U.S. Pat. No. 4,500,207 and Wong in U.S. Pat. No. 4,694,173 proposed NDIR techniques for gas detection without any moving parts such as mechanical choppers. The goal was to render NDIR sensors solid-state and hence more rugged and compact for use in a host of new applications.
Yamada in U.S. Pat. No. 4,605,855 further proposed an NDIR gas analyzer with a compact cell structure but retaining a mechanical chopper.
Kebabian in U.S. Pat. No. 4,605,313 proposed a new approach using an absorptive film on the surface of a thermal detector as a means to detect gases of interest. The behavior of the absorptive film specific only to the gas of interest directly modifies the detector output thus creating a unique signal. The reference is derived by using a chopper to detect a gas which does not affect the behavior of the absorptive film, thus generating a different signal at the same detector.
Even more recently Aoki in U.S. Pat. No. 4,662,755 disclosed the design of an NDIR gas analyzer with means for varying the angle of incidence of light on an interference filter in order to effect a reference and a sample path at two different wavelengths. Unfortunately, a mechanical chopper is again needed for implementing this particular design.
Despite these recent disclosures of new and simpler NDIR techniques for gas measurements, the goal of devising a uniquely simple device of small size and low cost, battery-operable, and with no moving parts has not been achieved until the presently disclosed invention.
The present inventor takes advantage of recent technological advances in infrared components. One area is the infrared source. The latest available device in this area is the so-called electrically modulatable infrared microsource which is, in essence, a small thick film resistor pad made out of special material capable of being heated and cooled at relatively high rate (up to 100 Hz, typically). This is achieved via standard I.sup.2 R pulsing using a square voltage waveform. Infrared micro-sources composed of a thick film of resistive material are available from Dynatech Electro-Optics Corporation of San Luis Obispo, Calif., and from Novametrix of Seattle, Wash. Micro-sources using a heated filament are available from Chicago Miniature Lamp Works of Chicago, Ill., and from Gilway Technical Lamp Co. of Woburn, Mass.
Another area of infrared components where recent technological advances make a significant impact is in narrow bandpass filters. Today not only can one make an excellent narrow bandpass filter with one center wavelength or pass band, filters with two narrow pass-bands centered at two spectrally separated wavelengths can also be routinely manufactured. Such a dual pass-band filter also plays a crucial part in the present invention. Dual pass band infrared filters are available from Barr Associates of Westford, Mass. and from OCLI of Santa Rosa, Calif.